Digital-to-analogue converters are known which comprise a voltage divider string in which each digital input word is assigned precisely one tap of said voltage divider string. The taps are switched to the output by means of an arrangement of analogue switches. A decoder drives the analogue switches in such a way that, for each value of the digital input word, the corresponding voltage is present at the output of the digital-to-analogue converter.
In integrated circuits, the voltage divider string is embodied as an extended resistive track on which lateral taps consisting of the same material as the resistive track are situated at regular intervals. At the end of the taps, contacts to metallic interconnects are situated at a sufficiently large distance, so that they have no significant influence on the profile of the current lines in the resistive track. By virtue of this arrangement, the homogeneity of the resistive track is no longer disturbed by the contacts whose contact resistances may have large fluctuations. The static linearity of this arrangement is determined only by the homogeneity of the resistive track and is generally very high. A significant disadvantage of this so-called R-string digital-to-analogue converter consists in the fact that the length of the resistive track increases proportionally to the number of taps. High-resolution digital-to-analogue converters therefore have a very long length or may no longer be able to be realized if their length would exceed the planned dimensions of the chip.
One known solution to this problem involves constructing a high-resolution R-string digital-to-analogue converter from two low-resolution R-string digital-to-analogue converters whose R-string resistors have different orders of magnitude. In this case, the high-resistance digital-to-analogue converters are always connected between two adjacent outputs of the low-resistance digital-to-analogue converter. The resolution of the digital-to-analogue converter realized in this way is the sum of the resolutions of the two underlying digital-to-analogue converters.
This method reduces the area requirement of the entire R-string digital-to-analogue converter. Since a high-resistance R-string with high linearity requires a great deal of area, overall only a relatively slight area gain can be achieved, however.